The present invention generally relates to fiber optical communication technologies and more specifically to a system and method for embodying amplitude information into phase masks for writing fiber Bragg gratings.
Normal optical fibers are uniform along their lengths. A slice from any one point of the fiber looks like a slice taken from anywhere else on the fiber, disregarding tiny imperfections. However, it is possible to make fibers in which the refractive index varies regularly along their length. These fibers are called fiber gratings because they interact with light like diffraction gratings. Their effects on light passing through them depend very strongly on the wavelength of the light.
A diffraction grating is a row of fine parallel lines, usually on a reflective surface. Light waves bounce off of the lines at an angle that depends on their wavelength, so light reflected from a diffraction grating spreads out in a spectrum. In fiber gratings, the lines are not grooves etched on the surface, instead they are variations in the refractive index of the fiber material. The variations scatter light by what is called the Bragg effect, hence fiber Bragg gratings (FBGs). Bragg effect scattering is not exactly the same as diffraction scattering, but the overall effect is similar. Bragg scattering reflects certain wavelengths of light that resonate with the grating spacing while transmitting other light.
FBGs are used to compensate for chromatic dispersion in an optical fiber. Dispersion is the spreading out of light pulses as they travel on the fiber. Dispersion occurs because the speed of light through the fiber depends on its wavelength, polarization, and propagation mode. The differences are slight, but accumulate with distance. Thus, the longer the fiber, the more dispersion. Dispersion can limit the distance a signal can travel through the optical fiber because dispersion cumulatively blurs the signal. After a certain point, the signal has become so blurred that it is unintelligible. The FBGs compensate for chromatic (wavelength) dispersion by serving as a selective delay line. The FBG delays the wavelengths that travel fastest through the fiber until the slower wavelengths catch up. FBGs are discussed further in Feng et al., U.S. Pat. No. 5,982,963, which is hereby incorporated herein by reference in its entirety.
In some applications it is desired to make FBGs which have multiple spectral bands of operation (channels). One method to make such devices is to further modulate the FBG with a period longer than the underlying grating period. This method of providing a superimposed structure or super-structure may sometimes be referred to as sampling. This super-structure may involve either modulation of the FBG amplitude or period (or phase). Examples of these type of FBG devices are described in the U.S. patent application Ser. No. 09/757,386, entitled xe2x80x9cEFFICIENT SAMPLED BRAGG GRATINGS FOR WDM APPLICATIONSxe2x80x9d.
FBGs are typically created in one of two manners. The first manner is known as the direct write FBG formation In this manner two ultraviolet beams may be impinged onto the fiber, in such a manner that they interfere with each other and form an interference pattern on the fiber. The interference pattern comprises regions of high and low intensity light. The high intensity light causes a change in the index of refraction of that region of the fiber. Since the regions of high and low intensity light are alternating, a FBG is formed in the fiber. The fiber or the writing system is moved with respect to the other such that the FBG is scanned, or written, into the fiber. Note that the two beams are typically formed from a single source beam by passing the beam through a beam separator, e.g. a beamsplitter or a grating. Also, the two beams are typically controlled in some manner so as to allow control over the locations of the high and low intensity regions. For example, Laming et al., WO 99/22256, which is hereby incorporated herein by reference in its entirety, teaches that beam separator and part of the focusing system is moveable to alter the angle of convergence of the beams, which in turn alters the fringe pitch on the fiber. Another example is provided by Stepanov et al., WO 99/63371, which is hereby incorporated herein by reference in its entirety, and teaches the use of an electro-optic module, which operates on the beams to impart a phase delay between the beams, which in turn controls the positions of the high and low intensity regions.
The second manner for creating FBGs uses a phase mask. The phase mask is a quartz slab that is patterned with a grating. This grating is typically a row of finely spaced parallel lines, or grooves, with a duty cycle typically in the forty to sixty percent range. These lines are usually etched lithographically onto the surface of the quartz slab (mask). The mask is placed in close proximity with the fiber, and ultraviolet light, usually from an ultraviolet laser, is shined through the mask and onto the fiber. As the light passes through the mask, the light is primarily diffracted into two directions, which then forms an interference pattern on the fiber. At this point, the FBG is formed in the same way as the direct write manner. See also Kashyap, xe2x80x9cFiber Bragg Gratingsxe2x80x9d, Academic Press (1999), ISBN 0-12-40056-8, which is hereby incorporated herein by reference in its entirety.
Each manner has advantages and disadvantages when compared with each other. For example, the phase mask manner, is relatively inflexible, as changes cannot be made to the mask. However, since the phase mask is permanent, the phase mask manner is stable, repeatable, and aside from the cost of the mask, relatively inexpensive to operate. On the other hand, the direct write manner is very flexible, and can write different gratings. However, this manner is less repeatable and is costly to operate.
When making an FBG there is a need to combine two pieces of information. A phase profile, provided by variation of the period, commonly referred to as chirp, of a phase mask and an amplitude profile (i.e. the magnitude of the index modulation of the core of the fiber) provided by varying the light exposure of the mask or other FBG creation mechanism. For complex FBG designs this creates an opportunity for errors. Assuming a perfect phase mask, manufactured in accordance with prior art methods, a phase mask only has a part of the information needed to write an FBG on a fiber, the phase information. The other information necessary for proper function of the FBG is the amplitude, which provides a profile for magnitude of a spatially varying oscillatory index modulation of the grating that is written into the core of the fiber.
In the prior art, the amplitude information is provided by separate data used to modulate the laser beam intensity or by other methods, such as rapidly vibrating the mask, fiber, or aiming mechanisms of the laser beam. Thus, in the prior art the amplitude information is controlled separately from the phase information that is incorporated into the mask. Therefore, great care has previously been required such that variation of the amplitude is controlled during the FBG writing process so as to be precisely spatially synchronized with the phase information incorporated into the mask in order to insure proper function of the grating. In addition, in the prior art, changes in the FBG amplitude made by modulating the laser intensity may cause changes in the average index of refraction of the fiber, which effectively leads to errors in the intended chirp.
One prior art attempt to combine phase and amplitude information for FBGs uses two masks. A phase mask and an amplitude mask are stacked or sandwiched. A window is cut out of a chrome layer of the amplitude mask. The light beam is focused through the amplitude mask and thus through the phase mask, focusing on the core of the fiber. This is undesirable as modulation of the mean index of refraction modifies the desired FBG. Ideally, a uniform mean index of refraction with symmetric oscillations of the index around the mean is needed to result in properly functioning FBGs. Therefore, under this prior art method a second pass of the laser beam after removing both masks and replacing the amplitude mask with a complementary mask amplitude pattern is necessary to equalize the mean index of refraction of the fiber. This method suffers from insufficient accuracy as a result of inherent inaccuracies associated with a multiple pass writing process.
It is also known in the art, for example as disclosed in Hill, U.S. Pat. No. 5,367,588, that the period of the grating on the mask preferably be chosen to be twice the desired period of the fringes in the FBG. This is because the fiber Bragg grating is formed by interference between the +1 and xe2x88x921 orders diffracted from the mask grating. The etch depth of the mask grating is chosen to suppress the zeroeth order. Accordingly, it is understood that any phase shift present in the mask is effectively doubled upon writing into the FBG. Thus, if a phase shift of xcfx80 or a half-period is desired in the FBG, then a phase shift of xcfx80/2 or a quarter-period is required in the mask grating.
Another prior art method to combine phase and amplitude information for FBGs uses modulation of the duty cycle, or etch depth, of the grating on the mask to modulate the visibility of the fringes in the transmitted light. This approach suffers from a number of practical difficulties in achieving desired flexibility and accuracy of the amplitude profile.
Using interference between two FBG fringe patterns to control fringe amplitude, referred to as apodization function, is disclosed in Kashyap, U.S. Pat. No. 6,307,679. However, the two component FBG patterns are written sequentially rather than simultaneously. As a result, the prior art method disclosed in Kashyap suffers from the problem that the longitudinal position of the fiber must remain very precisely controlled, generally on the scale of 1 nm, between the sequential writing passes of the two FBG patterns.
To solve the problems associated with separate amplitude and phase information masks, it is desirable to incorporate both parts of this complex function, the amplitude, and the phase function (variation of the grating period or chirp), into a single mask Thereby combining the phase and amplitude information so that it cannot be separated; eliminating a source of inaccuracy for FBGs.
The present inventive phase mask for writing FBGs in an optical fiber adjusts the mean index of refraction of the fiber that is being written on in a single pass. Specifically, the inventive phase mask embodies amplitude information into a phase mask so that the amplitude information is integral with the phase information. A first embodiment preferably employs an opaque surface layer that defines a window on the substrate of the phase mask to control the amplitude of the light passing through the phase mask. A second embodiment preferably employs a polygon shaped grating region on a smooth, clear substrate. A third embodiment preferably interleaves regions of grating and smooth substrate surface.
One preferred embodiment employs two areas of gratings with the grating in each area oriented perpendicularly. Another preferred embodiment employs two areas of gratings with one area disposed out of bandwidth for devices associated with the fiber optic system. A third preferred embodiment employs two areas of gratings with the areas disposed out of phase by a predetermined amount or with opposite phases, relative to each other.
Another preferred embodiment sums functions embodied by gratings disposed on a substrate to write a complex function in the fiber Bragg grating. One embodiment employs two halves of the complex function on left and right, and/or top and bottom halves of the phase mask.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.